This invention relates to a temperature sensing resistance device and particularly to a unique thick film resistor device for continuous monitoring of temperature.
In various monitoring and control circuits, continuous monitoring of a temperature condition may be required. For example, environmental controls for heating and cooling of buildings requires sensing existing conditions and developing control signals based thereon. Although various sensing devices have been proposed, the design of a device for precise continuous temperature monitoring at a relatively low cost has presented a continuous problem. Generally, the present designs are based upon one of three different temperature sensing elements including a: (1) thermocouple; (2) a thermistor and; (3) a resistance thermometer. A thermocouple which measures the temperature differential at the junction of two dissimilar metals provides an indirect signal, unless the one side of the junction is maintained and/or compensated to a reference temperature. Thermistors and resistance thermometers provide a direct indication of absolute temperature. Thermistors are generally semi-conductor devices having a high negative temperature coefficient of resistance. While thermistors avoid the objection of thermocouples, the device is relatively non-linear over many ranges of temperatures encountered in environmental and other controls and the response changes with time. Resistance thermometer devices are generally formed with a platinum element which functions with a high degree of accuracy and linearity. A thick film platinum element has recently and after the present invention has been made available by Engelhard Minerals and Chemical Corporation of New Jersey, (hereinafter referred to as Engelhard). Another device employing a bonded conductive track is available from Mathey Bishop, Inc. as thermafilm 100 R 30 sensors. The use of platinum, of course, results in a relatively high cost sensor and one which is also significantly vulnerable to mechanical damage.
There is a need, therefore, for a stable, long life and relatively low cost resistance sensor which produces a linear response over a relatively wide temperature sensing range such as encountered in many environmental and other control systems.